DALLAS (CBSDFW.COM) - Federal agents arrested two Dallas men Sunday after they watched them accept a shipment of 550 pounds of the drug Xanax, according to federal documents.

Brandon Lynell Gardner and William Ian Brown face federal charges of possession with intent to distribute a controlled substance.

According the federal court records, the truck driver assigned to make the delivery from Denver to Dallas didn’t know he was hauling drugs but had become suspicious of the content and contacted police.

Agents with U.S. Drug Enforcement Administration made the large Xanax bust near Northwest Highway and I-35 – five miles from Thomas Jefferson High School where earlier in the week the school called a parents meeting after a rash of Xanax overdoses on campus.

Last month the school called 911 on seven occasions because of prescription drug overdose concerns.

The Drug Enforcement Administration would not comment on Sunday’s arrests, but Dallas Division Assistant Special Agent in Charge Calvin Bond said in the past year federal agents have seen a dramatic increase in the number of prescription drug abusers in Dallas-Fort Worth.

He said the DEA has also noticed many of the prescription drug abusers have turned to heroin – causing a spike in the use of the illegal drug as well.

“The next thing you know somebody that you never thought would be sticking a syringe in their arm, they are doing it in a year and that’s terrifying.”

Bond said in many cases heroin is cheaper and easier to get than prescription drugs.

For the first time, he said, agents are seeing more heroin than marijuana coming up from Mexico.